THE SIR SUPPLIED DR. JAMES WITH 1G OF D, L-ARGININE-GUANIDO 13C, 15N2. ARGININE WITH 13C AND 15N LABELED IN THE GUANIDO GROUP IS OBTAINED FROM THE NATIONAL STABLE ISOTOPE RESOURCE AT LOS ALAMOS NATIONAL LABORATORY. THIS LABELED ARGININE IS INCORPORATED INTO A 25 AMINO ACYL PEPTIDE WHICH CONTAINS THE RNA BINDING DOMAIN OF THE TAT PROTEIN FROM HIV-1 AND HAS BEEN SHOWN TO PROMOTE HIV-1 REPLICATION CELLS. WHILE THE 25-MER HAS HAD ITS SOLUTION STRUCTURE DETERMINED VIA HOMONUCLEAR PROTON NMR STUDIES, THE SIDE-CHAIN PROTON RESONANCES OF THE SEVERAL ARGININES CANNOT BE RESOLVED IN THE SPECTRA. PLACING THE LABELED ARGININE AT THE POSITION CORRESPONDING TO R52 IN HIV-1 TAT IS IMPORTANT IS SUBSEQUENT STUDIES OF THE BIOLOGICALLY ACTIVE TAT PEPTIDE WITH TAR, THE RNA TO WHICH IT BINDS IN VIRAL REPLICATION. R52 HAS BEEN DEMONSTRATED TO BE ESSENTIAL FOR TAT ACTIVITY. THE STRUCTURE OF THIS SELECTIVELY LABELED PEPTIDE BOUND TO TAR SHOULD BE MUCH BETTER DEFINED BY USE OF THE LABELED ARG. CONSEQUENTLY, THE STRUCTURE DETERMINED SHOULD PROVIDE A BETTER TARGET FOR DEVELOPMENT OF A DRUG TO COMBAT AIDS.